brkyvz commented on code in PR #585:
URL: https://github.com/apache/parquet-format/pull/585#discussion_r3632340820
##########
LogicalTypes.md:
##########
@@ -635,7 +635,173 @@ The type has two type parameters:
The sort order used for `GEOGRAPHY` is undefined. When writing data, no min/max
statistics should be saved for this type and if such non-compliant statistics
-are found during reading, they must be ignored.
+are found during reading, they must be ignored.
+
+### FILE
+
+`FILE` annotates a group that represents a reference to a range of bytes,
which may
+be stored inline in the value, elsewhere within the current file, or in an
external file. It
+is intended for use cases such as storing file inventories, manifests, and
unstructured
+data references (e.g., images or audio files stored in object storage).
+
+The annotated group may contain the following fields, identified by name case
sensitively,
+not by field order. Field IDs, if they exist, may also be used for projection.
Every field
+is optional both in the schema and in the data: a writer may omit any field
from the group
+definition, and any field that is present has a field repetition type of
`OPTIONAL`.
+A group need only define the fields it uses (for example, an inline-only group
may define
+just `inline`, and an external reference may define just `path`).
+
+| Field | Type |
+|----------------|------------|
+| `path` | STRING |
+| `offset` | INT64 |
+| `size` | INT64 |
+| `content_type` | STRING |
+| `checksum` | STRING |
+| `inline` | BYTE_ARRAY |
+
+A value resolves to bytes determined by `inline` / `path` / `offset` / `size`;
+`content_type` and `checksum` are metadata describing whatever is resolved.
+
+#### Fields
+
+For the descriptions below, a field is *set* when it is present in the `FILE`
group
+and its value is non-null (and, for string fields, non-empty). A field is *not
set*
+when it is absent from the group, or is present but null or empty.
+
+##### path
+
+A URI-reference as defined by RFC 3986, encoded as a Parquet STRING (e.g.,
`s3://bucket/file.jpg`).
+The URI may be absolute or relative. No additional encoding (e.g., URI
encoding) is applied on top
+of the user-provided data. If `path` is not set, the value refers to the
current file
+(a self-reference).
+
+##### offset
+
+A byte offset indicating the start of the byte range within the referenced
data.
+If not set, readers must treat the value as 0.
+If set and non-zero, readers must seek to this offset to retrieve the
referenced data.
+`offset` must be set for a self-reference (`path` not set); it is optional for
an
+external reference (`path` set). `offset` must not be < 0.
+
+##### size
+
+The byte length of the referenced data. Must be zero or a positive integer if
set; a
+value of 0 indicates empty referenced data. `size` must be set whenever
`offset` is set.
+It may be omitted only for a whole-file external reference (`path` set,
`offset` not set),
+in which case the range runs to the end of the referenced file. Because a
self-reference
+always sets `offset`, it always sets `size` as well.
+
+##### content_type
+
+The media type (MIME type) of the resolved bytes (e.g., `image/png`).
+
+##### checksum
+
+A self-describing integrity token for the resolved bytes, of the form
+`<algorithm>:base64(<digest bytes>)`. Readers should ignore unknown
+algorithms. The recognized algorithms are:
+
+| Algorithm | Notes |
+|-----------|-----------------------------------------------------------------|
+| `ETAG` | the object-store eTag — equality-only, not recomputable |
+| `MD5` | the usual S3/HTTP eTag and Content-MD5 |
+| `CRC32` | Parquet's page-checksum algorithm (gzip/zlib) |
+| `CRC32C` | common in object stores, hardware-accelerated |
+| `SHA-256` | e.g. S3 additional checksums |
+
+`checksum` applies to the resolved bytes, except for `ETAG`, which is the
+object-store eTag for the whole file referenced by `path`.
+
+##### inline
+
+The referenced bytes stored inline in the value. If `inline` is set, it
supplies the
+bytes and any locator fields (`path`, `offset`, `size`) that are set are
provenance
+only.
+
+#### Resolution
+
+A value resolves to bytes based on which of `inline`, `path`, `offset`, and
`size` are
+set:
+
+| `inline` | `path` | `offset` | `size` | Resolves to
|
+|----------|--------|----------|--------|-------------------------------------------------------|
+| set | – | – | – | the inline bytes
|
+| – | set | – | – | whole external file at `path`
|
+| – | set | set | - | invalid
|
+| – | set | – | set | external `path`, `[0, size)`
|
+| – | set | set | set | external `path`, `[offset, offset +
size)` |
+| – | - | set | - | invalid
|
+| – | - | - | set | invalid
|
+| – | – | set | set | this file, `[offset, offset + size)`
(self-reference) |
+| – | – | – | – | nothing — invalid
|
+
+`size` must be set whenever `offset` is set, so any offset-based read always
carries an
+explicit `size`. A self-reference (`path` not set) must set `offset`, and
therefore also
+`size`. `size` may be omitted only for a whole-file external reference, where
the range
+runs to the end of the referenced file.
+
+A self-reference points within the same Parquet file using `offset` and `size`
(both
+required); the bytes are written between column chunks and are not otherwise
referenced by
+the footer. A self-reference is when `path` is not set, never an absolute path
back to
+the current file, so a file containing self-references is renamed or relocated
as a
+single unit.
+
+The bytes referenced by a self-reference are compressed with the same
`CompressionCodec`
+as the one specified for the `inline` column.
+
+#### Validation
+
+* A value must resolve to some referenced data. It resolves only if `inline`,
`path`, or
+ `offset` is set; if none of them are set, the value does not resolve and is
invalid, even
+ if `size` is set. Use column nullability to represent a null value.
+* A self-reference (`path` not set) must set `offset`. A value with neither
`path` nor
+ `offset` set (and not `inline`) does not resolve and is invalid.
+* `size` must be set whenever `offset` is set. A value that sets `offset`
without `size`
+ is invalid. Because a self-reference must set `offset`, it must also set
`size`.
+* If `inline` is set, it supplies the bytes; producers may instead treat
`inline` and the
+ locator fields as mutually exclusive.
+* Field names within a `FILE`-annotated group must not be renamed.
+* Additional metadata about the file (e.g., modification timestamp) must
+ be stored adjacent to this group by engines or table formats, not inside it.
+
+Statistics may be collected for the individual fields of a `FILE`-annotated
group
+according to the sort order of each field's logical type.
+
+This is an example of a `FILE`-annotated group that defines all fields:
+
+```
+optional group my_file (FILE) {
+ optional binary path (STRING);
+ optional int64 offset;
+ optional int64 size;
+ optional binary content_type (STRING);
+ optional binary checksum (STRING);
+ optional binary inline;
+}
+```
+
+Because every field is optional, a group need only define the fields it uses.
A group
+whose values are always stored inline may define just `inline`:
Review Comment:
I added ```and optionally `content_type` as additional metadata:```
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